UEFI, and Linux from first line to login

Learning goals

By the end of the week a student can:

• Explain what UEFI standardises, and why a generic distribution image boots unmodified on a board that implements it.
• Find the EFI system partition and the boot entries.
• Describe the kernel's boot: early console, device tree, driver probe and deferral, initramfs, root filesystem, init.
• Measure where a boot's time goes, and start a program at boot as a systemd service.

Lecture 1, block 1: what UEFI is

• A specification for the interface between firmware and an OS: storage, memory allocation, boot settings, and a few services that stay after the OS starts.
• Contrast with lab 4: there U-Boot was told exact files and addresses.
• U-Boot implements UEFI; both course boards keep a UEFI bootloader in SPI flash, so the boot media only carries the OS.

Lecture 1, block 2: the EFI system partition

• The OS's loader lives on a FAT filesystem, the EFI system partition.
• Firmware finds it through boot entries, or the fallback path \EFI\BOOT\BOOTAA64.EFI on 64-bit Arm.
• efibootmgr -v lists the boot entries from Linux.

Lecture 1, block 3: one image, many boards

• On Arm boards the firmware passes the device tree to the OS through UEFI too.
• Distributions ship one arm64 image for many boards; the firmware carries what is specific to the board.
• On Das Frite, which has no card slot, this is how a stock distribution boots from a USB drive.

Lecture 2, block 1: the kernel takes over

• An early console reports problems before the real serial driver is loaded.
• The kernel reads the device tree for memory, CPUs and peripherals, and starts the other cores through the secure monitor.
• Each driver is matched to a device-tree node by its compatible string and probed; one whose clock, regulator or GPIO is not ready is deferred and retried.

Lecture 2, block 2: root and init

• An initramfs can hold what is needed to find the real root filesystem.
• The kernel mounts root and starts init, process 1, which on most distributions is systemd.
• systemd starts every service, including the login prompt on the serial console.
• Kernel command-line arguments (read back in /proc/cmdline) change its behaviour without rebuilding it: U-Boot's bootargs in a hand boot, the distribution's boot-loader entry under UEFI.

Lecture 2, block 3: timing a boot

• The kernel's timestamps show where its time goes.
• systemd-analyze splits the boot, and its blame view lists the slowest services.
• A unit file with a restart policy is how a project starts on boot (week 9).

Board demo

1. Boot a stock generic arm64 distribution from a second boot media through the bootloader in SPI flash.
2. Show the EFI system partition, the fallback loader, and efibootmgr -v.
3. Run systemd-analyze and systemd-analyze blame, and read /proc/cmdline.